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From:   James Houghton <jthoughton@google.com>
Date:   Thu, 19 Jan 2023 15:06:27 -0800
Message-ID: <CADrL8HXHe85Jgiv9JatzMUhmqg0K5WGrGe_S+aZHobqzDYFLFQ@mail.gmail.com>
Subject: Re: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb
To:     Peter Xu <peterx@redhat.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        "Zach O'Keefe" <zokeefe@google.com>,
        Manish Mishra <manish.mishra@nutanix.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Yang Shi <shy828301@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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On Thu, Jan 19, 2023 at 2:37 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Thu, Jan 05, 2023 at 10:18:33AM +0000, James Houghton wrote:
> > +     /*
> > +      * Grab the VMA lock and mapping sem for writing. This will prevent
> > +      * concurrent high-granularity page table walks, so that we can safely
> > +      * collapse and free page tables.
> > +      *
> > +      * This is the same locking that huge_pmd_unshare requires.
> > +      */
> > +     hugetlb_vma_lock_write(vma);
> > +     i_mmap_lock_write(vma->vm_file->f_mapping);
>
> One thing I just noticed - do we need the mmap write lock here?  I don't
> quickly see what stops another thread from having the mmap read and walking
> upon the pgtables being collapsed.

Maybe. Does huge_pmd_unshare() have the same problem?